The present invention relates to a semiconductor whisker, a semiconductor device using the same and a manufacturing method thereof, and more particularly to a semiconductor device having channels such as a field effect transistor (FET), a ballistic transistor and the like, a semiconductor device having a needle-pointed electron emitting material such as a semiconductor vacuum microelectronic device and the like, a semiconductor device having a quantum wire such as a light emitting element and the like, and a manufacturing method thereof.
FIG. 47 and FIG. 48 show sectional views of typical field effect transistors (hereinafter referred to as an FET) using a compound semiconductor which has been heretofore used. A case in which GaAs is used principally as a compound semiconductor will be explained hereafter with reference to FIG. 46 and FIG. 47.
In an FET shown in FIG. 46, an n-type GaAs layer 4 is laminated first on a GaAs semi-insulating substrate 7, and an ohmic source electrode 5, a drain electrode 3 and a gate 2 for forming a Schottky junction are formed thereon. A width of a depletion layer 9 is varied by changing a gate bias, thereby to control a three dimensional electron gas (3 DEG) flowing through a channel. Such an FET is called a MESFET in general.
In an FET shown in FIG. 47, a first semiconductor layer 4 composed of an undoped GaAs layer is formed on a GaAs semi-insulating substrate 7, and an n-AlGaAs layer 10 is formed further thereon. Then, highly doped layers S and D are provided while being separated from each other extending from the surface to the semiconductor 4, and a source electrode 5 and a drain electrode 3 are provided through ohmic junction on the surface with these doped layers S and D as a source and a drain. A two dimensional electron gas (2 DEG) produced along an interface between the undoped GaAs layer 4 and the n-AlGaAs layer 10 is used as a channel and controlled by a gate bias. Such an FET is generally called a high Electron Mobility Transistor (HEMT). With the change of the device structure from a MESFET to a HEMT, it has been noticed that electron mobility in the channel is improved remarkably and the operation speed of the FET is also improved. Further, in the HEMT itself, with the improvement of the quality of the crystal, the electron mobility has been increased year by year. However, the value of the electron mobility has also reached the upper limit at present, and transconductance has reached the upper limit already, too, in accordance with the above. There is such a problem that the operation speed of an FET can not be increased any more because the transconductance of a present FET has an upper limit as described above.
While a demand for high speed performance of a device is increasing, an FET having a new structure is necessary in order to break through this limit. As an FET for meeting such a demand, an FET using such a fine wire that electrons conduct themselves as a one-dimensional gas has been proposed as a channel in J.J.A.P., vol. 19, No. 12 (1980) pp. L735-L738. When electrons running with momentum hk through a wire in a wire direction are scattered elastically by means of ionized impurities and the like in a fine wire confined with two dimensions, with the energy preserved low or the energy remained constant the state of electrons after scattering only has hk momentum. Since such elastic scattering is accompanied by large momentum variation, it is foreseen that an elastic scattering rate is very small. Accordingly, it is expected that electrons have very large electron mobility in such a fine wire. Further, if this fine wire is used as a channel, high transconductance may be expected because the electron mobility is high, thus making it possible to realize an FET which is superior in high-speed performance. As described above, several attempts have been made to form a fine wire in which electrons are confined with two dimensions, what is called a quantum wire, but such a technology that a two dimensional gas region formed with a HEMT structure is formed into a wire by nanofabrication is principally adopted. Therefore, damages induced during fabrication cannot be avoided. Thus, an FET having a quantum wire channel of high quality has not yet been obtained.
Incidentally, the electron mobility of GaAs at room temperature is at 8,500 cm/V.s, but the mobility is lowered as a practical matter by means of impurity scattering because impurities are doped when a device is manufactured.
The electron mobility in a channel having a donor impurity concentration at 1.times.10.sup.18 /cm.sup.3 is at approximately 2,000 cm/V.s and lowered to 1,000 cm/V.s and below at the base, etc. of a bipolar element having an impurity concentration at 1.times.10.sup.19 /cm.sup.3 and higher, and the time required for electrons to pass through an operation layer gets longer by the ratio in which the mobility is lowered, hence the operation speed of the element is reduced. As a means for improving such drawbacks, a ballistic transistor has been proposed in Electronics Letters vol. 16 (1980) pp. 524-525. FIG. 48 shows an example of a device structure thereof. Since the mean free path of electrons in a GaAs crystal is approximately 200 nm in the case of an undoped crystal, electrons can pass through the channel at a high speed without being scattered when a channel length is limited to 400 nm or less. This high speed electronic conduction is referred to as ballistic conduction. When doping of impurities is performed for the purpose of reducing channel resistance, it is necessary to shorten the channel length by the ratio that the mean free path of electrons is reduced in keeping with doping.
A conventional semiconductor vacuum microelectronic device is described in "OHYO BUTSURI" (Applied Physics) vol. 59, No. 2, pp. 164-169, 1990.
FIG. 49 shows in schematic section a structural view of above-mentioned Si vacuum microelectronic device. The construction is composed of an emitter 101 composed of Si formed by utilizing anisotropic wet etching on a (100) plane of a Si substrate 100, an insulator 102 provided around the emitter, a gate 103 and an anode 104, and element operation is performed based on the same principle as a three polar vacuum tube. Here, since electrons emitted from the emitter 101 drift in a vacuum until reaching the anode, some electrons travel faster than the drifting speed of electrons in a semiconductor in principle depending on the distance and the applied voltage between the anode and the emitter. For example, when a potential difference at 50 V is applied between two sheets of platelet electrodes placed in parallel with each other at an interval of 1 .mu.m, electrons travel at a mean velocity of 2.times.10.sup.8 cm/sec. and the travelling time is 0.5 picosecond. Thus, a super high speed microelectronic device on the order of terrahertz may be realized with a microelectronic device having a dimension on the order of a micron. Such a super high speed microelectronic device cannot be realized in a conventional FET and hetero junction bipolar transistor (HBT) in which electrons drift in a semiconductor material. This is because of such reasons that the drifting speed of electrons in a semiconductor material is determined by a saturated speed and does not exceed a value roughly of 2.times.10.sup.7 cm/sec.
A conventional quantum wire is disclosed in "Applied Electronic Material Property Sectional Committee, Research Report No. 425, pp. 11-16 published by The Japan Society of Applied Physics on Sept. 16, 1988".
The above-mentioned quantum wire will be explained with reference to FIG. 50. FIG. 50 is a schematic diagram showing in section a structure of a GaAs quantum wire. As it is seen in FIG. 50, the GaAs quantum wire is formed inside a plurality of crystal layers composed of GaAs layers 322 and 325 and AlGaAs layers 323 and 324.
This quantum wire is formed by making a peripheral portion of a two dimensional electron gas region utilizing hetero junction between GaAs and AlGaAs to have a high resistance by focused ion beam implantation into Si. In FIG. 50, a region having a very small width W equal to approximately 0.1 .mu.m which is not contained in a focused ion beam implanted portion 326 shows a quantum wire among a two dimensional electron gas region shown at 327.
In Appl. Phys. Lett. 51 (1987) pp. 1518-1520, a method of forming a GaAs quantum wire by utilizing selective growth is disclosed. FIG. 51 is a schematic diagram showing a sectional structure of above-mentioned GaAs quantum wire. As shown in FIG. 51, the GaAs quantum wire is formed inside a structure composed of a plurality of layers, GaAs layer 333 and AlGaAs layers 334 and 335.
In the above-described quantum layer, a two dimensional electron gas region 336 is formed by selective growth of GaAs and AlGaAs on a substrate 331. At this time, it is possible to make the height H of the undoped GaAs layer 333 small by making the width D of the selective growth portion approximately 0.1 .mu.m, thereby reducing the region of the two dimensional electron gas 336. Thus, a quantum wire is obtained.
Reduction in line spectrum width of an oscillation light and lowering of threshold current density have been realized in recent years by using a semiconductor quantum well and a quantum wire in an optical device such as a semiconductor laser unit and by multi-dimensional quantum confinement effect of carriers. This fact brings about a preferable result as a light source of high performance in optical communication for instance.
A variety of manufacturing methods have been proposed for such quantum wells and quantum wires. In particular, a semiconductor quantum wire is manufactured by applying nanofabrication technology such as etching and the like to principally a semiconductor such as GaAs (Appl. Phys. Lett. 41 (1982) pp. 635-638).
However, a quantum wire produced by such nanofabrication technology has a drawback that a plurality of damages are formed on a surface or an interface of the fine wire. Such a damage becomes a nonluminescent center in case of a photo emission mechanism and lowers the luminous efficiency substantially. Accordingly, even if a semiconductor laser is produced for instance by utilizing such a quantum wire, highly efficient oscillation can not be generated, thus causing a fatal drawback in practical applications.